Crystal oscillator circuit for tetrodes



April 23, 1935.

A. H. TAYLOR CRYSTAL OSCILLATOR CIRCUIT FOR TETRODES Filed June 15, 1933n m 2 u a w A m a n m i u M W 9L: 1 4 mmg m m 5 B m E n mi 19 h u m +6 uD) u g a m L W M; u n H w w m H E m u H u n u nvvElv'ro'R' M Albert H.Ta ylor- Patented Apr. 23, 1935 :iy 'f d.

1,998,469 v a CRYSTAL OSCILLATOR cincurr FOR x -rrnrnonns r I AlbertTaylor, Washington, D. 0. Application June 15,1933, Serial No. 675,958 1claim; (01;; 250-36) f (Granted "underlthejactof March; a, 1883, asamended Apt-i130, 1928; 3'10 0. G. Z 57) This invention I relates tocrystal controlled vacuum tube oscillators, particularly those. designedto function in the type ofgcircuit known to the art as the masteroscillator-power amplifie circuit. 7

An object of this invention is to produce a crystal controlledvacuumtube :oscillator that is substantially unaifected in frequency bychanges in an exteriorly connectedload circuit,

such as a power amplifier; 7 a,

A further object of this invention ispto supply a crystal-controlledvacuum tube circuit utilizing) a four electrode-shield grid typeof,vacuum A still further object of thisinvention is .to provide a crystalcontrolled vacuum'-tubeloscillator having a greater degree of inherent.frequency stability than heretofore obtained in similar circuitsutilizing three-electrode vacuum tubes.

invention consists in the construction, combination and arrangement ofparts as will be hereinafter more fully described.

Reference is to be had to the accompanying drawing forming a part ofthis specification which illustrates by a schematic wiring diagram thepreferred embodiment of my invention.

In the usual design of master oscillator-power amplifier circuits asused in the radio art, it is well known that changes in or adjustmentsof the amplifier are reflected back into the oscillator, producingfrequency changes of greater or less degree depending on thecharacteristics of o the oscillator, the type of tube used, and thegeneral design of the circuit.

I have found that such reactions of the amplifier circuit tuning on thefrequency of the crystal controlled master may be materially reduced 40by a special form of coupling to the amplifier circuit when the mastercrystal controlled tube is a tetrode. The drawing shows a tetrode, whichin this case is a 7 /2 watt shield grid tube, with the crystal connectedin the usual way between control grid and ground, the pointcorresponding to ground being the mid-tap of the transformer secondary 3supplying voltage to the filament.

The screen grid of this tube is not used as a screen but is utilized asa plate or anode whose circuit is tuned by means of a variableinductance 4 and capacity 5 and preferably includes a radio frequencyammeter A. This tunable circuit is grounded through a condenser I2 of.01

microfarad at a point intermediate between the ends of the inductance 4.Adjustable on the in- With the above and. other objects in view, the

ductance l thereisalso connected a balance condenser 6 which is led backto the normal plate of-the oscillator tube. a

This crystal circuit, when supplied with voltage-on the screenonly,goscillates.in theusual 5 manner when the output circuit, 4 and 5,connected to the screen, is-properly tuned to the crystal frequency..Atthe same time, the balance condenser 6 can be adjusted in pick-01fand capacity so as tosupplyareversed electric coupling to the plate,exactly-neutralizing the-e1ec-' trio coupling betweenscreen and plate.Thus thereis neither magnetic nor, electrical coupling,

v between the tuned screen circuit and the'plate.

The plate is connected through a stopping 15 condenser 1 tothe grid [5of the amplifier tube which may be operatedeither as a straightamplifieror as afrequency doubler, as desired. a

When there is no voltage present on the oscillator. tube plate as whenthe switch I3 is open, and no magnetic nor electrical coupling betweenthat plate and the crystal oscillator circuit, the grid I5 of theamplifier tube will receive no excitation. By means of a potentiometer 8placed across the high voltage supply for the crystal oscillator tube,various potentials may be supplied to the screen and plate of that tube.These potentials can be then adjusted until a certain percentage of theelectron stream will flow through the screen grid and strike the plate,thus giving an adjustable coupling to the grid of the amplifier tube.This coupling is what may be called an electron stream coupling todistinguish it from a coupling by means of a variable magnetic orelectric field of the usual type. This adjustable coupling, by means ofthe electron stream which flows through the meshes of the screen,enables one to get a crystal controlled oscillating circuit which issubstantially independent of the changes in load or tuning of theamplifier system. 40

It will be noted that the working grid of the amplifier tube is providedwith a choke l l and is also negatively biased. A sensitive directcurrent instrument H! in series with the negative lead to thisamplifiergrid indicates whether or not that grid has excitation. If,then, when the plate supply of the first tube is disconnected, leavingonly the screen with high voltage, the balance condenser 6 is adjusteduntil the direct current meter [4 reads zero, the circuit is balanced,that is, the 0 electric coupling within the crystal controlledoscillator tube between screen and plate has been neutralized and anysubsequent coupling must be by means of. the electron stream which flowsthrough the meshes of the screen. The amount of this electron streamcoupling can naturally be regulated by the voltage on the plate of theoscillator tube which can be at a slightly higher voltage than thescreen but more often is considerably lower than said screen.

Ordinarily, for simplifying operation, the voltage to the plate of theoscillator tube is fed through a radio frequency choke 9 but the outputof the circuit, that is to say, its ability to deliver excitation to theamplifier grid I5, may be greatly enhanced, as shown in the diagram, bysubstituting for the radio frequency choke 9 a parallel tuned circuitIn. It has beenfound, however, that in this case the freedom fromreaction is not .quite as good as if the choke 9'were used.

It is very important that the differentparts of this circuit be keptscreened from each other, as shown by screen It. In particular, the mainoscillator circuit must be screened-from the parallel tuned inductanceH! in the plate of that same tube, if such be used in place" of thechoke 9. Both of these circuits must be screened from the output of theamplifier-tube if that tube is amplifying at the same'frequency. This isnot important, however, if the second tube is used as afrequency'multiplier. I Y

It is obvious'fro'mthe foregoing that with this particulartetrode'balanced circuit and by means of its electron stream coupling, agreater stability of thecrystal oscillator circuit is obtained, togetherwith an improved degree of freedom from tuning and load reactance in theamplifier circuit, Thispermits a strong coupling to be used betweenmaster oscillator and amplifier without the consequent reaction havingan influence on the frequency. While itis realized that no crystalcontrolled circuit is seriously in fiuenced in frequency by tuning orload reaction in a system-amplifier, nevertheless, for work of highprecision the matter is at times troublesome.

We are dealing here with an effort to improve small variations infrequency of the order of 0.01%.

It will be noted that in this particular circuit one side of the crystalholder can be kept at ground potential, simplifying the design when amultiplicity of crystals are to be used in the transmitter. In suchcases it is highly desirable to keep the base plate of the crystals atground potential in order that all crystals may be mounted on a commonmetal plate acting as a heat equalizer within the temperature controlbox.

It will be understood that the above description and accompanyingdrawing comprehend only the general and preferred embodiment of myinvention, and that various changes in construction, proportion andarrangement of parts may be made within the scope of the appended claim,and without sacrificing any of the advantages of my invention.

The herein described invention may be manufactured and used by or forthe Government of the United States for governmental purposes withoutthe payment to me of any royalties thereon.

I claim:

In an oscillation generator, a four-electrode vacuum tube having acathode, a control grid, a grid-like anode, and a plate, a frequencydetermining control grid circuit, an oscillatory circuit, a directcurrent conductive path between a first point on said oscillatorycircuit and said grid-like anode, a variable capacitative connectionbetween a second point on said oscillatory circuit and said plate, and afixed capacitative connection between a third point on said oscillatorycircuit and ground, said third point being intermediate said first andsecond points.

ALBERT H. TAYLOR.

